If you play standard MSS on 6-max, it's not profitable, because blinds pass too often. Your stack gets eaten up by the blinds. At 6-max game is also a lot looser/more aggressive.

Playing MSS on 6-max is possible, but you need to deviate from standard MSS. Your ranges need to be a lot looser, and you really need a good postflop game. Your raises and 3-bets will get called more often.